[Bill Thomas]

Friday, October 30th, 2009 - Update:  "And In the End..."

     Well, we're not *quite* to the end of our journey, but we certainly ARE getting close!  All of the "signal" tube sockets have now been wired.  Here's a picture of the final tube socket used in the Dynaural noise reduction circuitry:

                       

     Unlike the 6AV6 socket, most of the components for this part of the circuit are mounted on the terminal strip immediately below the socket.  Still, there were a *few* parts surrounding the tube socket.  A couple of hours of checking, re-checking, re-re-checking and THEN soldering, and we're gettin' close to the final wiring on the Main Chassis.

     The High Voltage Power Supply sections are also now complete.  Here's a picture of the wiring around the two final quad capacitors:

           

     You'll notice that two sections of the right-hand quad cap are still yet to be wired.  They are part of the DC filament supply; which is the final stage of wiring on the Main Chassis, along with the installation of the Power Transformer.  Here's a picture of the area we'll be focusing on next:

           

     While the 6X4 rectifier socket looks pretty vacant, all but two of the socket lugs are fed directly from the Power Transformer.  The other two pins are used as "tie points" for the filament supply components. Since they aren't internally connected inside the 6X4, using them this way is just FINE!

     By the way, that two-lug terminal strip will also be wired when the Power Transformer is installed.  Those terminals are used to terminate the two wires from the Power Switch on the Front Panel.

     While there IS more to come, take a look at our *nearly* completed Main Chassis:

     

     Other than the filament supply, the Power Transformer and the wires that lead to the back chassis, we're REALLY close to the completion of our "project."  (I can feel the anticipation building!)  Don't go away!  We'll be installing our "beautified" Power Transformer next!  Stay with us!

Sincerely,

Bill Thomas

[Bill Thomas]

Tuesday, November 3rd, 2009 - Update:  "Closer To (its NEW) Home"

     With apologies to Grand Funk Railroad, here are a couple of pictures of our Power Transformer in "ready to install" state.  It was pretty breezy and dusty when this was shot, so my apologies for the "particles" on the cover.  It actually looks a good deal prettier than the pictures show.  First, here's a picture from the side.  This shows the copper eddy-current strap reflecting the grain of the wood the transformer is sitting on:

                                   

     And here's a picture from the front of the transformer.  The red highlights are actually reflections from the shirt I was wearing:

                 

     While I mentioned we would be installing the transformer next, I have decided to install the rear panel first.  Otherwise our nice, new paint job might get damaged.  With the rear panel installed and the chassis inverted, the Power Transformer won't rest on the surface of the bench.  While the new paint on the transformer cover is dry, it won't achieve maximum hardness for about a month.  I'd just rather not tempt fate by resting the unit on the Transformer cover at this stage.

     The Rear Panel will require a good deal of work before it's "ready to go."  We still have seven potentiometers to clean and lubricate, as well as whole BOATLOAD of RCA jacks to clean and de-oxidize.  Since my "French Maid" isn't skilled in the ways of H. H. Scott, (Don't ask me what she IS skilled at!  I'm still trying to figure THAT one out!), I'll have to tackle the chores myself!

     Just to show you what's next, here are a couple of pictures of the Rear Panel in "as removed" condition.  First, here's a picture of the side everyone will see from the back of the preamplifier:

     

     Notice that there are a couple of "stickers" on the back.  The foil "H. H. Scott" sticker will *probably* survive just fine!  But the paper "patent notices" sticker will be a bit more problematic.  All I can say is, I'll do my best to save it!

     Now, here is a picture of the side most folks WON'T see, the INSIDE of the Rear Panel:

     

     LOTS of dust and dirt to remove, as well as a few "stragglers"; a few small parts that remain to be replaced.  The "stragglers" are the least of our worries.  They'll all be replaced with new parts.

     Sorry about the poor lighting of the pictures.  Even at High Noon, the sun is now VERY low in the sky so there are LOTS of shadows to deal with.  But this will at least give you an idea of what is yet to come.

     We'll cover the panel cleanup and installation, the Power Transformer installation, the remaining installation of the DC Filament Supply and the incoming Power wiring in our next update.  At THAT point, we'll be *just* about ready to "fire her up!"  We'll STILL have another "Update" after that.  In that final construction update, we'll show you the installation of the two cleaned up cover/shields for the front and back panels, the (cleaned up) Front Panel, the nice, clean knobs on the controls and we'll take care of one more *teensy* little detail - a cosmetic part that was missing when I received the unit.  Details, details.  But as they say, "The Devil is in the DETAILS."  Let me tell you, they KNEW what they were talking about!

     Our next update will follow VERY soon.  We really ARE "Getting Closer to Home" on this one now.  I can almost SMELL the anticipation building!  (No wait, I *think* that was the CAT!)  Anyway, until our NEXT update, stay warm and stay tuned!

Sincerely,

Bill Thomas

[Bill Thomas]

November 29th, 2009 - Update: - "I'm Headin' For That Last Roundup!"

     And "hell-bent for leather" to get this thing completed by TOMORROW!  As things stand, the chassis is now completely electrically assembled.  Let's take a look.

     Last time, we showed you the inside of the rear panel in "as-found" condition.  Now, take a look at the pre-wired rear panel with the new parts installed:

               

     I had planned to re-use the original "Level Trim" potentiometers, but one of them had a fractured element that worked *most* of the time.  Sorry, but that just won't do.  We're after a RELIABLE preamplifier, so all six of the "Level Trim" potentiometers were replaced.  The 100K "Cartridge Loading" potentiometer was in FINE shape so it stays.  Here's a picture of the pre-wiring around the new potentiometers:

               

     Scott used bare wires from the potentiometers to the Input Jacks, but to make things a bit neater, I used the same color-coded wires that are used in the shielded multi-conductor "tube" that runs from the Front Panel area.  No chance of an inadvertent short here!

     Now, here's a picture of the pre-wiring at the low-level Input Jacks:

                             

     And finally, here's a shot of the pre-wiring around the AC supply area:

                                       

  New fuse holder, new line cord but we DID retain the original "convenience receptacles."  They are in FINE condition so there was no reason to change them.

     We still had a bit of work to do on the main chassis before we could "button it up" and call it done.  It all hinges on the installation of the cleaned-up Power Transformer.  Well, here's the result:

               

     Looks pretty nice, doesn't it?  It certainly beats the original appearance of the Power Transformer by a LONG shot!  Remember what THAT looked like?  No?  Well, take a look:

                   

     With the Power Transformer mounted, it was time to complete the wiring of the filament circuitry and the wiring around the High Voltage rectifier, the 6X4.  Here's a picture:

               

     In the final analysis, we only changed the values of two resistors in the DC filament supply.  Originally, the value of the two current-inrush limiting resistors was a total of 5.4 Ohms.  Since we increased the size of the capacitors in the filament supply, I thought it *might* be a good idea to increase those resistors a bit.  Now, instead of 5.4 Ohms, they are a total of 7.6 Ohms.  As it turns out, this little change was all that was necessary.  The 33 Ohm dropping resistor remained the same as the original value.  With the three tubes installed, the Voltage delivered to the series string filaments is 17.6 Volts.  This is within 1/10th of a Volt of the original Voltage value of 17.5 Volts.

     You might wonder why H. H. Scott ran these tubes below their normal "rated" Voltage.  According to the tube specifications, these tubes SHOULD be running at a total of 18.9 Volts, so what gives?  Simple!  By running the tubes at a *slightly* lower filament Voltage, tube life increases DRAMATICALLY!  This also explains the 2.7 Ohm resistor in series with the tubes fed with AC from the Power Transformer.  Since there is gain to spare, running the tubes a *teensy* bit below their nominal Voltage rating extends the life of the tubes.

     With THAT out of the way, let's wire in the rear panel!  Here's a picture of the wiring to the "Level Trim" potentiometers:

               

     It's pretty hard to show the rest of the wiring to the rear panel, so let's wind things up with an "underside" shot of the completed preamplifier:

               

     And finally, here's a picture of the top side of the completed wired preamplifier:

               

     Oh!  I *almost* forgot!  I mentioned there was one little "detail" we had yet to cover.  Well, here it is - the "jewel" for the pilot lamp:

               

     No, this isn't an original jewel, but it captures the "flavor" of the LONG extinct original jewel pretty well.  It's installed the way the original one was mounted - glue!

     While there are still a few "Final Assembly" details yet to go, they are of a "cosmetic" nature.  In the final "Update", we'll show you the completed unit with all shields in place.  We'll also mount the face plate to the front panel, add all the knobs and then we'll "go for the gusto" with the "smoke test."  Be still my beating heart!!!

     As I mentioned, I'm shooting for a completion date of TOMORROW, so don't make any plans for tomorrow night!  We're going to hear this baby in operation!  Stay with us!

Sincerely,

Bill Thomas

[smbrown]

I felt this all needed a comment, but the only thing I could think of was, Wow! 

[Bill Baker]

Absolutely stunning work Bill.....as always

[Bill Thomas]

Monday November 30th, 2009 - Update:  "Problems, Problems!"

     Thanks for the kind comments, guys.  Right about now, I need all the "positive waves" I can find.  We have a problem and it's NOT going to be an easy fix!

     I always tell folks there are TWO ways I do things; LIKE an idiot, or AS an idiot.  It looks like I have BOTH bases covered with THIS little *error.*  But first, let's look at the GOOD news.

     We've had out initial "smoke test" and nothing exploded.  The missing 6AV6 has arrived so all tubes are present and accounted for.  Upon initial application of power, there was a TRUCKLOAD of noise.  It was traced to a bad (and microphonic) 12AX7.  A NOS RCA gray plate solved THAT "issue" with a minimum of "fuss."  I also replaced the other 12AX7 with another NOS RCA gray plate "just to be sure."  For testing purposes, I use a little Sony portable CD player; not because it has FABULOUS sound quality, but because it is battery powered and has a Line-level Output.  This eliminates any possibility of Power Line-induced hum.  Of course, I initially forgot all about the little back panel "Level Trim" controls, so I DID waste about ten minutes trying to figure out why I wasn't hearing anything.

     Advancing the Level Trim control for the "Spare" Input, (and plugging the CD player INTO the "Spare Input"), I was rewarded with the UNMISTAKABLE sound of Tom and Ray Magliozzi, laughing their heads off.  (OBVIOUSLY, I grabbed the wrong CD!)  Nevertheless, our little Scott preamplifier was "doing the job" quite nicely.  There's a *little* bit of hum, but since it changes as I reposition the unit on the bench, I'm chalking it up to the missing shields and the missing bottom cover.  Bringing my hand near the 12AX7 Output Tube, the hum increases DRAMATICALLY!  I guess there really IS a reason why Scott used a tube shield at this location.  Needless to say, I will ALSO put a shield on the two tubes that require it when we get to the final testing phase.

     OK, now that we have audio, I tried each of the Line-Level Inputs.  Yep!  They ALL worked flawlessly.  The "Level Trim" controls are noise free!  (Whew!)  Checking the Volume and Tone Controls, they all function normally.  So far, so good!

     The "star" of the show in this preamplifier is the "Dynaural Noise Reduction" circuitry.  It is designed to minimize "impulse noise" on old 78's (or trashed LP's).  Since I was dealing with a (relatively) noise-free source, I could only verify that the Rumble Filter was working properly and that the different bandpass settings of the Dynaural Selector Switch were having an appropriate effect.  Well, they did indeed!  And the Dynaural Level Control also operated noiselessly.  It certainly appears that the Line-Level circuitry, the Dynaural Noise Reduction circuitry and the Tone Control circuitry are all doing what they are supposed to do!

     Felling pretty good at this point, I switched the unit to one of the Phono Inputs and used the "fingertip method" to check for a signal from the Mag 1 Input.

     Nothing!  Not ANYTHING!  Not a TRACE of any hum or noise of ANY kind!  I tried the other Magnetic Phono Input - Mag 2.  Again, NOTHING!  I switched the Main Selector Switch to the NARTB Tape Input and tried the "fingertip test" again.  THIS time, I was rewarded with a LOUD hum when my fingertip touched the Input jack.

     Digging a bit deeper, I turned the unit upside-down and applied my fingertip (VERY carefully) to the Input point of the Equalized Preamplifier tube (a 12AU7).  With the Selector Switch set for the NARTB Tape position, I was again rewarded with a LOUD hum.  But when I switched the Selector Switch to either of the Phono Inputs, I got NOTHING!  This Input point occurs AFTER the Selector Switch, so I *should* get a rip-snortin' hum!  OBVIOUSLY there is something that is shorting out this point whenever the Selector Switch is set to the Phono positions.

     After spending several HOURS tracing ALL of the wiring to the Selector Switch, nothing revealed itself.  All of the wiring to the switch was found to be correct.

     But the GOOD news is, all "active" gain stages of the preamplifier are working, we just have a little *problem* lurking SOMEWHERE around the Main Selector Switch.  But WHAT???  Could the switch be defective?  Could a wire be touching somewhere around the Phono Section of the switch?

     Nope!  In the immortal words of "Pogo", "We have met the enemy and he is US!"  Using an Ohm meter, I was able to ascertain that the wiper of the switch was making contact with the correct terminals.  (This chewed up several MORE hours of troubleshooting.)  Still, after all this, NOTHING was found to be remiss.  All of the wires are connected to the appropriate terminal numbers of the Selector Switch.

     We've all experienced the "Eureka Moment!"  That's the point where the light bulb over our head lights up with the dawning of realization.  In MOST cases, this is a VERY good thing!  In THIS case, it confirmed my worst fear - I had goofed when re-assembling the Main Selector Switch!  HOW I did this, I will NEVER know, but nevertheless, I DID do it!

     In this case, I had the wipers of the two switch sections properly aligned, but one set of stationary contacts was rotated 180 degrees from where it SHOULD be.  It's the only POSSIBLE explanation for this problem!  This is the point where my own words have come back to haunt me:  "It is all too easy to "goof" and re-assemble the switch with the wafers incorrectly oriented."  The trouble is, this little "goof" will require unsoldering ALL of the wires to the Front Section of the Selector Switch, dis-assembling the switch AGAIN and rotating the front wafer 180 degrees and then re-soldering ALL of those wires!

     And that's where things stand right now.  I have begun the LONG and ARDUOUS process of attempting to get to the switch and FIX my stupid mistake!  Since it is COMPLETELY buried, this is no EASY fix, but with care, patience and persistence, we'll EVENTUALLY get things where they SHOULD be!

     Like an idiot?  Or AS an idiot?  YOU be the judge!

     If anything can be learned from this, it is this:  No matter how silly you think it is to check and RE-CHECK your work, the fact is, you can NEVER check it TOO many times.  Check EVERYTHING and check it over and over until you are absolutely CERTAIN you have things "in order."  Even then, don't be surprised if, after all this checking and re-checking, you find something wrong after assembly.  When that happens, take a break.  Have a drink!  Go watch something mindless on the tube.  Whatever!  Just get AWAY from it for a bit.  Then, when you come back to it, use your BRAIN to determine the cause of the problem and take some time to think about *exactly* what course of action you are going to use to FIX the problem.  Be creative!  But BE THOROUGH!

     Well, we've taken a bit of a detour, but we *should* be back on the "straight and narrow" path within a couple of days.  I'll keep you posted!

Sincerely,

Bill Thomas

     

[Hank]

Bill, consider this a VERY positive wave!    You do great, meticulous work.  I've got 4 Scott integrated tube amps and 2 Scott tuners and would like for you to work your magic on a couple of them, but I don't dare get a quote from you, based on the number of hours I assume you put into your work.  I've got one of the amps almost completely re-capped and have replaced critical resistors and will continue on with it.

Your caution to check and re-check is a good reminder for us all.  I would add that the mental attitude to do your checking in should be that of TRYING to find something wrong rather than trying to confirm you did perfect work.  With the latter 'tude, I've missed mistakes.  With the former, I've not missed mistakes.
Keep on truckin'!

[Bill Thomas]

     Thanks, Hank!  I TOTALLY agree with you regarding the idea of looking for something WRONG, as opposed to confirming that something is RIGHT!

     I'm "digging out" the Selector Switch as we speak.  It *looks* like we're going to be doing quite a bit of "re-working and re-wiring" before all is said and done.  I have un-bolted the Front Panel from the Main Chassis and "tilted" it slightly to allow access to the selector switch.  Next, I have separated the rear wafer, the spacers and the shield, but access is EXTREMELY tight.  It appears that I'm going to have to un-solder three connections on the rear wafer in order to swing it out of the way to allow access to the front wafer.  Once I can do THAT, there are nine connections that must be un-soldered from the front wafer.  This is a real BEAR of a job.  Best to consider this a "learning experience." in order to keep me from kicking myself around the block for a while!

     Once this is finished, I will finally finish off our second FM-3 tuner.  There is a problem with THAT little beastie that has kept me scratching my head, off-and-on, for the past few months.  I have found that once I get too intensely involved in tracing a problem, it becomes necessary to get away from it for a while in order to face it with a "fresh brain."  The FM-3 plays, but it isn't possible to align it properly.  The problem is on the new IF board *somewhere* and I'm comparing a working tuner to this one.  It helps to have a working tuner in order to inject "known good signals" from one tuner into the other one.

     After this, we'll do a nice, easy, simple Stereo 70 restoration.  I think I'm gonna need a little rest before I tackle another H. H. Scott of ANY sort!  (BOY did they like to use LOTS of parts!)

     How many hours?  That's a question for the ages.  I'm not doing this as a "money-making" operation so I'm not keeping track of the total number of hours "invested" in these rebuilds, but if I did, I'll bet I would be working for a LOT less than minimum wage! (lol)  But these are not "cheap and dirty" repair jobs.  As always, my goal is to give these old and tired pieces a "second lifetime" of reliable operation.  "Quick and dirty" has NEVER appealed to me.  I think people are getting tired of the "quick and dirty" approach.  The downside of this is that it takes a LOT of time to complete each unit.  I wish I worked faster, but I would rather have it done RIGHT than done FAST!

     Well, back to the salt mines!

Sincerely,

Bill Thomas

     

[Hank]

Yes, Scott used lots of parts because Scott over-engineered their products, but that's why they sound great today    Oh yeah, I also have my original Dynaco AF-6 which needs an alignment since I have no such experience nor proper equipment. 
I agree with you on the number of hours thing.  I've never kept track when building speakers for me or the occasional customer; otherwise I too would be working for less than minimum wage

[Bill Thomas]

December 13, 2009 - Mini Update:  "Right or Wrong?"

     After carefully UN-building a significant portion of our preamplifier in order to get down to the selector switch, I discovered that I DIDN'T assemble the selector switch with a wafer 180 degrees off.  In other words, my only real goof (so far) was dis-assembling this thing to get down to the selector switch.

     What I DID discover (and the thing that has had me going around in circles for several days) is that my limited documentation has errors in their depiction of the selector switch!  These errors show a switch that couldn't POSSIBLY work correctly which led me to believe that I had rotated one of the wafers.  While we still DO have a problem, it is NOT in the selector switch area.  It is apparently in the variable equalization circuitry that goes between the selector switch and the second stage of audio amplification.  I'm guessing that our "homemade" repair on the 1 Meg potentiometer may not be working properly.  While I work out the "issues" on THAT little area, I wanted to make SURE folks knew about the errors in the documentation that wasted two weeks of my time.

     My limited documentation is a (poor) photocopy of a Howard W. Sams photofact schematic.  The H. H. Scott 121-C is covered in Photofact number 380, folder 10.  While the *orientation* of the selector switch is shown correctly, the actual switch contacts are NOT depicted properly.  Several of the "short" and "long" contacts are out of place.  I can only thank God (and my retentive NEED to take LOTS of pictures) for FINALLY confirming the documentation errors.

     The GOOD news is the selector switch IS wired correctly.  The BAD news is that it will take several days to get everything back where it was before I had to dig down DEEP to get to the selector switch!  Such is life.

     I hope to be able to report on the ACTUAL problem by tonight.

Sincerely,

Bill Thomas

[Bill Thomas]

December 14th, 2009! - Update:  "At Last!"

     (The Etta James version, of course!)  We FINALLY have a working Phono Section!  In other words, we are DEFINITELY heading down the "home-stretch" of this most *interesting* project!  The problem with the Phono section was FINALLY traced down to the "home-brew" repair of the potentiometer.  Oh, the pot itself was working just FINE!  But there was one TINY little detail that caused a problem.  Here's a picture to help everyone see what finally caused the problem:

                                       

     If you'll look CAREFULLY at the protruding screw threads from that TINY little screw holding the wiper contact surface in place, you'll see the "offender!"  As it turned out, once the control was re-assembled, mounted and the nut holding the control to the panel was tightened, this little protruding thread made intermittent contact with the rear of the Front Panel.  A few minutes with a file eliminated the excess threads AND the problem!

     In troubleshooting ANY piece of electronic equipment, "intermittents" are the most time-consuming, (expletive-deleted), drive-you-crazy types of problems.  As it turned out, simply "nudging" the preamp on the bench would make this "short" come-and-go.  Of course, I *thought* I had checked the clearance, but there was *just* enough extra "give" in the front of the metal framework of the potentiometer for the screw thread to make *slight* contact.

     I just thought you all would like to know what the FINAL cause of the problem was.  I'll have a wrap-up report on the finished unit coming up as soon as my final testing is completed.

     More to come (with pictures, no less.)

Sincerely,

Bill Thomas

[Bill Thomas]

Friday, December 18th, 2009 - Final Update:  "Goodbye Yellow Brick Road!"

     As I type this, our H. H. Scott 121-C is about ten hours into the "burn-in" process.  Our little trip down the "Yellow Brick Road" has FINALLY come to an end!  It is making some pretty music and *appears* to be functioning properly and reliably!  But BOY, has it taken a long time to get to this final stage!

     ALL of these restoration projects take on a life of their own, but THIS one threw a LOT of "challenges" in our path!  Having to dis-assemble a significant part of the unit just to find out that I *didn't* make a mistake was probably the biggest "buzz kill" of the whole project, but it simply HAD to be done due to the "documentation errors" in the Howard W. Sams Photofact schematic.  We live and learn!

     OK, let's get down to business by looking at some photos of the final assembly stages of the "project."  First, here's a "View From the Top" of the completely wired unit prior to the addition of the chassis shields:

               

     While I *usually* HATE having reflections of my arm holding the camera, in THIS case it helps to show how much nicer the chassis turned out than the corroded mess we started out with!  Remember how things looked when we began?  Here's a little "reminder":

               

     While nothing is ever *perfect*, I'd say we're in MUCH better shape now than when we started this journey!  At this stage, the final metal parts hadn't been installed yet, but the bare front panel looked like this:

               

     You'll notice that we had to "elongate" the mounting holes for the replacement Power Switch a bit.  Other than that, it all looks pretty "stock", but there's a LOT going on *behind* that stock-appearing Front Panel.  A "Three-Quarters" shot from the rear of the preamplifier helps to "fill in the blanks" a bit:

               
               

     A "Three-Quarters" shot from the Front of the preamplifier shows a bit more detail of the Line Level circuitry on the Rear Panel:

               

     I must admit, having individual "Trim" controls for the Line-Level Inputs is a pretty nice feature.  Coming from a background in Recording Studios, I REALLY appreciate the ability to properly match the levels of the various Line-Level sources!  This comes in ESPECIALLY handy when dealing with a CD player that is putting out over 2 Volts of audio, compared with a "standard" Line-Level source that "only" supplies ONE Volt of audio!  (By the way, there is an honest-to-goodness REASON why today's CD players supply Two Volts of audio!)

     When the CD system was originally "envisioned" it was assumed that CD's would be issued with a 6 dB "headroom margin" and you will often find MANY early CD's that adhere to this "standard" arrangement.  In *that* case, a CD with a 1 kHz tone recorded at -6 dB from absolute MAXIMUM level will output a 1 Volt audio signal (or thereabouts) from the Line-Level Output jacks!  Of course, as CD's production progressed, engineers found that the 6 dB headroom was unnecessary so they "cranked up volume" to the max!  (Personally, I think this was *probably* a step in the wrong direction, but it CAN help to improve the low-level "definition" of the recorded audio on a CD.  It just doesn't allow for "overshoots" through the analog portion of the CD player.)

     Anyway, let's get back to our little preamp!  Here's a picture of the underside of what I *thought* was the completely wired preamplifier:

               

     This picture shows the preamplifier in "totally stock" condition.  As it turns out, "totally stock" wasn't completely acceptable.  Here's why:

     I mentioned that this unit is EXTREMELY sensitive and susceptible to hum pickup, but what I DIDN'T expect was an excessive amount of 120 Hz ripple from the Power Supply using "stock" capacitance values.  While it *might* have been acceptable in 1957, I consider it "borderline" when compared with other preamplifiers of the day.  Clearly, we needed to add more filtering!  Thanks to the smaller size of modern electrolytics, we were able to add two more capacitors underneath the chassis.  Here's a picture:

               

     There was *just* enough room available to "beef up" the Power Supply to bring the 120 Hz ripple to acceptable levels - well BELOW the original published specifications for this preamplifier.  I didn't want too much capacitance across the Output of the 6X4 rectifier; THAT could shorten the life of the tube.  Instead, I added 47 microfarads to the SECOND stage of the filtering and I added an additional 10 microfarads to the third stage that supplies the Output section of the preamplifier.

     With THAT little problem resolved, it was time to "button her up" by adding the rest of the "cosmetic" parts and the shields.

     The Bottom Cover is an ESSENTIAL part of the overall shielding of the preamplifier and it turned out a little nicer than I expected.  Here's a picture:

               

     There were a couple of "dings" that I wasn't able to *completely* remove, but all things considered, it's not TOO shabby!  (Besides, this is a part of the preamplifier that isn't *normally* "on display" anyway, right?)

     Now, here's a picture of the Rear Panel with all of the newly cleaned knobs properly in place:

               

     I did my BEST to try to "save" the little paper "patent notice" label, but it DID suffer a bit of damage during the rear panel cleanup process.  I guess you simply can't win them all, can you?  Still, it was necessary in order to fully clean up the RCA jacks and remove some interesting "stains" that appeared to be from a spilled soft drink.

     Now, here's the part EVERYONE will see the most - the Front Panel, with all of the knobs installed and properly indexed:

               

     While there are a couple of small scratches that have occurred somewhere during the past 52 years of this preamplifier's lifetime, I think things turned out pretty nicely, don't you?

     And our FINAL shot shows the front and rear "cover shields" that serve to "button up" the exposed Front and Rear panels:

               

     These shields are also an ESSENTIAL element needed to keep hum and noise pickup to a minimum.  Unfortunately, I don't have the "proper" tube shields "in-stock" for this unit.  Instead, I have added a couple of shields from my stock of Dynaco FM-3 tuner shields.  They "do the job", but they are *obviously* not "correct" for this preamplifier.  The shield on the left covers the 12AU7 used in the Low-Level Phono stage and the tube shield on the right covers the 12AX7 Output tube.  BOTH shields are absolutely ESSENTIAL to keep hum and noise to a minimum!

     That about "covers" it!  (Sorry!  I just couldn't resist the bad pun!)  I'll discuss the operation of the unit next.

Sincerely,

Bill Thomas

[Bill Thomas]

Friday, December 18th, 2009 - Update:  Part Two!

     Let's discuss a few additional points:

     Since I had NO "audio reference point" when this project began (after all, ALL of those "bumblebee" capacitors and ALL of the electrolytic capacitors were totally toasted!), there was NO way I could make a "before and after" audio comparison.  I CAN report that the completed preamplifier sounds VERY nice and there is NO question that the variable equalization circuitry adds an INCREDIBLE amount of flexibility necessary for the proper playback of early 78's.  The "Dynaural Noise Reduction" circuitry goes a LONG way to help reduce surface noise from old shellac records too.

     The variable "Turnover" and "Rolloff" potentiometers operate quite differently.  The "Rolloff" control has a great deal of effect on the audio, while the "Turnover" control can best be described as "subtle."  I also noticed that the audio level of the "Variable Equalization" setting of the Main Selector Switch is substantially lower than the "Fixed RIAA EQ" setting.  The schematic diagram also shows this to be the case.  I assume this is to keep the extreme settings of the EQ controls from overloading the Low-Level circuitry.  Whatever the reason, it works!  The Phono Preamplifier stage has the ability to impart a "full-bodied" quality to even acoustically recorded 78's!  The Tone Controls also operate VERY well.  The shelving frequencies were well-chosen and the amount of boost and cut is VERY helpful!  The "Loudness" contours add a bit of boost to both the Low AND High frequencies and under the right low-level listening conditions, they are quite "pleasant" (Unlike many other "Loudness" contours I can name.)

     The variable loading for the "Mag Phono 1" Input is a welcome addition and can help "tame" cartridges that have a particularly high Output Level.  That's a GOOD thing since this little preamplifier is VERY sensitive!  In the "Fixed RIAA Equalization" position, I found that a cartridge with a very high output CAN overload the Phono Preamp stage.  (The cartridge I tested this with has an EXTREMELY high Output when compared with most other cartridges out there.)  With a Shure V-15 type IV-G cartridge, there was no trace of any overload.  (The Shure V-15 type IV-G cartridge was supplied by Neumann with their VMS-70 disc cutting lathes in the 1980's in order to verify proper frequency response from a cut lacquer.  I just happened to have one left in my "parts stash.")

     It should be mentioned that this is NOT the *quietest* preamplifier I have ever heard.  But it is EASILY one of the finer preamplifiers of its time.  The amount of flexibility available is simply AMAZING when compared with more "conventional" preamplifiers with "fixed" equalization curves.

     Somewhere, I hope Hermon Hosmer Scott has a smile on his face, knowing that one of his "children" has been brought back from the dead!

                                                       

     (This image of H. H. Scott was "borrowed" from the EXTREMELY useful  H. H. Scott Hi-Fi Stereo Archive.  Visit http://www.hhscott.com/ for MUCH more information.)

     Well, that's about it!  I hope this has been an interesting journey back to the early days of "Hi-Fi."  It has certainly been *quite* an "education" for ME!  We've covered some of the "pitfalls" of restoring a VERY difficult preamplifier to rebuild.  *I* have learned a LOT through the "process" myself!  I have CERTAINLY learned that with enough time, patience and persistence, almost ANYTHING can be accomplished - even a "home-brew" rebuild of a now extinct potentiometer!

     This project has also increased my appreciation for the SIMPLICITY of Dynaco's offerings when compared with the UNBELIEVABLE complexity of this unit!  Two VERY different paths to the same overall goal - TRUE audio excellence!

Sincerely,

Bill Thomas

[Hank]

Kudos and admirations, Bill!      Outstanding work!  She's a beauty now!

[Bill Thomas]

     Thanks, Hank!  I'm just glad to FINALLY be able to call this one finished!  I DO think it turned out quite "pretty" from a cosmetic standpoint as well.  (It wasn't exactly a "dog" in the looks department when I started, but all the visible metalwork *did* seem to clean up pretty well.  That aluminum chassis was ANOTHER story!  It was VERY oxidized with a good deal of pitting and corrosion.  Without stripping the chassis COMPLETELY, it would have been absolutely IMPOSSIBLE to get it even *this* nice.

     And a sincere "Thank YOU" for your encouragement along the way (as well as everyone else here for theirs too!)  When a project like this runs "into a wall" the encouragement helps to motivate me into seeking an "alternative", but "acceptable" solution!  THIS thing hit more walls than Spiderman did during his first day of web-slinging!

     Ahhh, but the GOOD thing is we are now nearly 24 hours into the "burn-in" phase and it sounds *quite* lovely.  All controls are quiet, the Power Transformer is nice and "tepid" to the touch and the music still sounds lovely!

Whew!

Sincerely,

Bill

[Bill Thomas]

Sunday, December 20th, 2009 - Update:  "No News is GREAT News!"

     "Burn-in" continues with absolutely ZERO problem!  The music sounds great and the preamplifier is running cool and comfortable to the touch.  No hints of any Voltage on the chassis.  In other words, all is well!

     I'll give it another full day of "burn-in" and then give it one final checkout of all inputs and controls.  After that, it's going to its new home for the holidays!

Sincerely,

Bill Thomas

[Listens2tubes]

Superb as always Bill. Thanks for all your excellent work. Happy Holidays

[hifigeezer]

Hi, I just came across this awesome thread!  The pictures are not visible -- can they be turned back on please?

Thanks for posting!

[SteveFord]

Unfortunately, Mr. Thomas has passed away and I don't have the capabilities to locate the missing pictures as I'd like to see them, too.

[Mark Korda]

Hi HiFi  Geezer,Bill Thomas was the heart and soul of this Vintage catagory.His Dyna rebuild articles are suberb,ask Frank VanAlstine who knows!He died not to long ago,but when I had a printer with my computer,he sent me a message that encouraged me so much,it hangs on my wall,art! I don't know who the moderater here is,but not showing Bills photos are like supressing Ansel Adams pictures of the west...Bill told us all he brought his pictures,that I think are fanstasic,into real sunlight,outside....the Audio Circle should put those H.H. Scott pics on!.not throw into the fire a great library of know how....Mark Korda